A manuscript should be proofread at least twice—once at the very end of the writing/editing process before the book is designed and a second time after the book is designed and typeset. Years ago, books were always proofread by at least two experienced proofreaders, increasing the odds of catching all the errors that hide in a manuscript.
It is virtually impossible for a writer to proofread his or her own work and hope to find all the errors. Once you are familiar with a manuscript, your eyes see what we think we wrote, not what is actually on the page.
Proofreading is not another editing step—it is primarily intended to catch formatting errors, such as imperfect spacing, and perhaps a few punctuation errors or typos that might have been missed by the copy editor.
You can expect to spend at least $25-$55 an hour for a professional proofreader, probably $500 to $1,000 in total for your manuscript, depending on its complexity.
This is your last opportunity to check the formatting, page order, and everything else before your book is sent to the printer. You may be able to do this yourself, but if you are printing a large quantity, in particular, it is a good idea to hire a new proofreader, someone who hasn’t seen the book before, to do one last check.